Active and Passive Voice

There are two ways to express an action of a subject in relation to its object : 

• Active voice

• Passive voice

In “active voice” subject acts upon object, while in “passive voice” object is acted upon by subject. It can also be said, in “active voice” subject does a work on object, while in “passive voice” object is worked on by subject. The normal structure of an active voice sentence is subject+verb+object but in passive the normal structure of sentence is reversed according to certain rules and becomes like object+verb+subject. The real meaning of a sentence does not change if the sentence is expressed either by active voice or by passive voice. The active voice is mostly used in writing because it gives a direct and more concise meaning. Passive voice is used sometimes due to the following reasons.

1. When intentionally hiding the subject of sentence. For example, a student who failed in exam might say, some chapters were not studied. Another example. Women were not treated as equals
2. When passive voice better explain thought of sentence. For example, to say, cloth is sold in yards, is more meaningful than to say, Shop keepers sell cloth in yards.
3. When passive voice better emphasizes the main though of the sentence. For example, a man who is being teased by another person might say in anger, “you will be beaten by me”
4. When subject is not exactly known. For example, His watch was stolen. It is not known that who stole his watch, the subject (thief) is not exactly known so it is better to use passive voice for such sentence.There are certain rules for expressing a thought in passive voice or for changing a sentence from active voice to passive voice.

ACTIVE / PASSIVE VOICE

Active voice
In most English sentences with an action verb, the subject performs the action denoted by the verb.
    These examples show that the subject is  doing the verb's action.

            

Because the subject does or "acts upon" the verb in such sentences, the sentences are said to be in the active voice.

Passive voice
One can change the normal word order of many active sentences (those with a direct object) so that the subject is no longer active, but is, instead, being acted upon by the verb - or passive.
    Note in these examples how the subject-verb relationship has changed.

            

Because the subject is being "acted upon" (or is passive), such sentences are said to be in the passive voice.
NOTE:   Colorful parrots live in the rainforests cannot be changed to passive voice because the sentence does not have a direct object.
To change a sentence from active to passive voice, do the following:
    1.  Move  the active sentence's direct object into the sentence's subject  slot

             

    2. Place the active sentence's subject into a  phrase beginning with the preposition by

              

    3.  Add a form of the auxiliary verb be to the main verb and change the  main verb's form

              

Because passive voice sentences necessarily add words and change the normal doer-action-receiver of action direction, they may make the reader work harder to understand the intended meaning.
As the examples below illustrate, a sentence in  active voice  flows more smoothly and is easier to understand than the same sentence in  passive voice.

             

            

It is generally preferable to use the ACTIVE voice.

To change a passive voice sentence into an active voice sentence, simply reverse the steps shown above.
    1.  Move  the passive sentence's subject into the active  sentence's  direct object slot

             

    2.  Remove  the auxiliary verb be from the main verb and change main verb's form if needed

            

    3. Place the passive sentence's object of the preposition by into the subject slot.

           

Because it is more direct, most writers prefer to use the active voice whenever possible.
The passive voice may be a better choice, however, when

• the doer of the action is unknown, unwanted, or unneeded in the sentence

             Examples

             

• the writer wishes to emphasize the action of the sentence rather than the doer of the action

            Examples

               

• the writer wishes to use passive voice for sentence variety.

Source :

http://www.towson.edu/ows/activepass.htm

http://www.studyandexam.com/active-passive-voice.html